For various lighting applications, different colored light-emitting diodes (LEDs) can be combined to provide light with a certain desired color point. LEDs can emit green, blue, yellow or red light in the visible portion of the light spectrum. It is possible to obtain a ‘white’ LED by using a phosphor to convert the light emitted by a blue LED into white light. Similarly, blue light from blue-emitting LEDs can be converted into red light using suitable phosphors. However, this less efficient compared to directly obtaining red light from red-emitting LEDs, since phosphor conversion is associated with Stokes shift losses. With suitable combinations of ‘white’ and colored LEDs, light with certain efficacy can be achieved. For example, ‘white’ LEDs can be combined with red LEDs to obtain light with a reddish hue. Such a combination can be useful when it is desired to enhance the color rendering of red or reddish objects such as certain foodstuffs, such as meat, fruit, red-colored vegetables, etc. since the color rendering of red objects using ‘pure’ white light is generally poor. The color point of a combined-color light source can be determined by the number of white and colored LEDs used, and/or by the manner in which they are driven. For example, in one approach, the contribution of a particular LED color can be increased or decreased by adjusting the nominal forward current for the LEDs of that color. Alternatively, additional LEDs can be activated in order to obtain a desired overall color.
In any diode, the P/N junction activation energy levels exhibit temperature dependency, which can be quantified as a relationship between junction temperature and the drop in diode forward voltage. This relationship will depend to a large extent on the material used for the diode. For this reason, different colored LEDs have different temperature characteristics, and the luminous or photon flux of an LED, and therefore the light output of the LED, is therefore temperature-dependent. In other words, the light output of an LED starts to drop above a certain temperature. Therefore, the color point of a light source using LEDs of different colors will shift away from the initial color point. For example, red LEDs in combination with LEDs of one or more different colors (such as white) will be shifted ‘away’ from the red as the temperature increases. This may be problematic, since the human eye is very sensitive to slight color changes, i.e. to slight variations in color point. In a lighting application in which the red component is important, for example for refrigerated display lighting or shelf lighting, such a shift in color point can be noticed and may have an adverse effect on the perceived quality of the lighting. The light output of LEDs that emit light within a certain color range, for example ‘red’ LEDs (wavelength about 660 nm) and ‘far-red’ LEDs (wavelength about 730 nm), can also differ significantly as the junction temperature rises. While imperceptible to a human observer, the relationship between red and far-red components of the light spectrum can have noticeable effects on plants that are illuminated by a lighting arrangement using a combination of such LEDs, since plant phytochromes require a balance between red and far-red light, and plant physiological processes such as blossom induction, stem stretching, germination etc. are largely controlled by the plant phytochromes.
To deal with the problem of color point ‘drift’ with rising temperature, combined-color LED lighting arrangements generally make use of some kind of sensor to detect the temperature of the LEDs and/or to sense the color of the light output by the LEDs. For example, a prior art lighting arrangement using a temperature sensor can determine when a certain junction temperature has been reached, and can drive the more temperature-sensitive red LEDs by increasing their LED current. Another prior art lighting arrangement uses an optical color sensor, for example a tri-color or bi-color photodiode array, to continually monitor the combined color output. To correct a color shift or color point drift, a color feedback control circuit can be used to drive LEDs of the ‘weaker’ color so that these contribute more to the overall light output. Again, this can be achieved by increasing the LED current of those LEDs or by activating more LEDs of that color. Another known approach is to include a voltage measurement means in the circuit to measure the drop in forward voltage over temperature-sensitive LEDs such as a string of red LEDs. The forward voltage drop value is then converted by a controller into an increase in current for those more temperature-sensitive LEDs.
However, such sensors or measurement circuitry are expensive and add to the overall cost of the lighting arrangement. If ‘extra’ LEDs are included to compensate for a possible color shift at higher temperatures, these also add to the cost of the lighting arrangement but are only used during a color adjustment at such high temperatures, and are otherwise unused.
US2011/0115406 describes a white light emitting device with a primary and secondary set of LEDs, in particular a set of red and a set of blue lights. The device includes a drive circuit to compensate for variation in the ratio of red to blue light in the emission product due to the operating temperature. The circuit does not compensate for the emission intensity. The driver applies temperature sensors and/or temperature dependent resistors.
Therefore, it is an object of the invention to provide a more economical combined-color LED lighting arrangement with a favorably constant color output.